Radiation is often separated into two categories, ionizing and non-ionizing, to denote the energy and danger of the radiation. Ionization is the process of removing electrons from atoms, leaving electrically charged particles (ions) behind.

Many forms of radiation such as heat, visible light, microwaves, or radio waves do not have sufficient energy to remove electrons from atoms and hence, are called non-ionizing radiation. In the case of heat, for objects at room temperature, most of the energy is transmitted at infra-redwavelengths.

The negatively charged electrons and positively charged nuclei created by ionizing radiation may cause damage in living tissue. The term radioactivity generally refers to the release of ionizing radiation.

Radioactive materials usually release alpha rays (particles similar to the nuclei of helium), beta rays (quickly moving electrons) and gamma rays. Alpha and beta rays can often be shielded by a piece of paper or a thin sheet of steel. They cause most damage when they are emitted inside the human body. Gamma rays are less ionising than either alpha or beta rays, but protection against them requires thicker shielding. They produce damage similar to that caused by X-rays such as burns, cancer, and genetic mutations. Human biology resists germ-line mutation by aborting most mutated conceptuses.

The earth, and all living things on it, are constantly bombarded by radiation from space, similar to a steady drizzle of rain. Charged particles from the sun and stars interact with the earth's atmosphere and magnetic field to produce a shower of radiation, typically beta and gamma radiation. The dose from cosmic radiation varies in different parts of the world due to differences in elevation and the effects of the earth's magnetic field.

Radioactive material is found throughout nature. It occurs naturally in the soil, water, and vegetation. The major isotopes of concern for terrestrial radiation are uranium and its decay products, such as thorium, radium, and radon. Low levels of uranium, thorium, and their decay products are found everywhere. Some of these materials are ingested with food and water, while others, such as radon, are inhaled. The dose from terrestrial sources varies in different parts of the world. Locations with higher concentrations of uranium and thorium in their soil have higher dose levels.

In addition to the cosmic and terrestrial sources, all people also have radioactive potassium-40, carbon-14, lead-210, and other isotopes inside their bodies from birth. The variation in dose from one person to another is not as great as the variation in dose from cosmic and terrestrial sources.

Natural and artificial radiation sources are identical in their nature and their effect. Above the background level of radiation exposure, the NRC requires that its licensees limit man-made radiation exposure to individual members of the public to 100 mrem (1 mSv) per year, and limit occupational radiation exposure to adults working with radioactive material to 5,000 mrem (50 mSv) per year.

The exposure for an average person is about 360 millirems/year, 81 percent of which comes from natural sources of radiation. The remaining 19 percent results from exposure to man-made radiation sources.

By far, the most significant source of man-made radiation exposure to the general public is from medical procedures, such as diagnostic X-rays, nuclear medicine, and radiation therapy. Some of the major isotopes would be I-131, Tc-99m, Co-60, Ir-192, Cs-137, and others.

Of lesser magnitude, members of the public are exposed to radiation from the nuclear fuel cycle, which includes the entire sequence from mining and milling of uranium to the disposal of the used (spent) fuel. The effects of such exposure have not been reliably measured. Estimates of exposure are low enough that proponents of nuclear power liken them to the mutagenic power of wearing trousers for two extra minutes per year (because heat causes mutation). Opponents use a cancer per dose model to prove that such activities cause several hundred cases of cancer per year.

In a nuclear war, gamma rays from fallout of nuclear weapons would probably cause the largest number of casualties. Immediately downwind of targets, doses would exceed 30,000 roentgen/hr. 450 R (more than a thousand times the background rate) is fatal to half of a normal population. No survivors have been documented from doses above 600 R.

Occupationally exposed individuals are exposed according to their occupations and to the sources with which they work. The exposure of these individuals to radiation is carefully monitored with the use of pocket-pen-sized instruments called dosimeters.
Some of the isotopes of concern would be cobalt-60, cesium-137, americium-241, and others. Examples of industries where occupational exposure is aconcern include:

We tend to think of biological effects of radiation in terms of their effect on living cells. For low levels of radiation exposure, the biological effects are so small they may not be detected. The body repairs many types of radiation and chemical damage. Biological effects of radiation on living cells may result in four outcomes:

Low levels of ionizing radiation may be beneficial to many types of cells; this phenomenon is termed radiation hormesis[?], see below.

The associations between radiation exposure and the development of cancer are mostly based on populations exposed to relatively high levels of ionizing radiation (e.g., Japanese atomic bomb survivors, and recipients of selected diagnostic or therapeutic medical procedures).

The period of time between radiation exposure and the detection of cancer is known as the latent period. Those cancers that may develop as a result of radiation exposure are indistinguishable from those that occur naturally or as a result of exposure to other chemical carcinogens. Furthermore, National Cancer Institute literature indicates that other chemical and physical hazards and lifestyle factors (e.g., smoking, alcohol consumption, and diet) significantly contribute to many of these same diseases.

Although radiation may cause cancer at high doses and high dose rates, public health data do not certainly establish the occurrence of cancer following exposure to low doses and dose rates -- below about 10,000 mrem (100 mSv).

Most studies of occupational workers exposed to chronic low-levels of radiation above normal background have shown no adverse biological effects. Even so, the radiation protection community conservatively assumes that any amount of radiation may pose some risk for causing cancer and hereditary effect, and that the risk is higher for higher radiation exposures.

The linear dose-response model suggests that any increase in dose, no matter how small, results in an incremental increase in risk. The LNT hypothesis is accepted by the NRC as a conservative model for estimating radiation risk.

High radiation doses tend to kill cells, while low doses tend to damage or alter the genetic code (DNA) of irradiated cells. High doses can kill so many cells that tissues and organs are damaged immediately. This in turn may cause a rapid whole body response often called Acute Radiation Syndrome. The higher the radiation dose, the sooner the effects of radiation will appear, and the higher the probability of death.

This syndrome was observed in many atomic bomb survivors in 1945 and emergency workers responding to the 1986Chernobyl nuclear power plant accident.

Approximately 134 plant workers and firefighters battling the fire at the Chernobyl power plant received high radiation doses (70,000 to 1,340,000 mrem or 700 to 13,400 mSv) and suffered from acute radiation sickness. Of these, 28 died from their radiation injuries.

It has never been proven that very low does of ionizing radiation are harmful. A small but growing number of studies offer evidence that these does may have some beneficial effects.

A linear, no-threshold (LNT) dose response relationship is widely assumed to be valid by most policy makers and many scientists. However, a small but growing number of scientists hold that this relationship is grossly misleading, and may be totally wrong. One problem is that this relationship ignores known cellular repair mechanisms; cells in all organisms have efficient methods to detect and repair damage.

Some scientists hold that these linear-response curves were created with an anti-nuclear political and social agenda in mind, and have little or no scientific validity.

It is easy to show the fallacy of the linear no-threshold relationship: Background radiation in our everyday environment does not kill people, yet radiation blasts from nuclear fission events (e.g. worst-case meltdowns or nearby atomic bombs) can almost immediately kill a person. These deadly radiation events are nearly a million times more powerful than background radiation. Compare this to taking one aspirin a day (which we may call background level); this has been proven to be harmless for most people, and actually has substantial medical benefits for many people. If one were to take one million aspirin a day, that person would die immediately. The same is true of most essential vitamins and minerals; small amounts are harmless, or even necessary for life. Doses a million times larger are not healthy, and potentially fatal. No scientist would make a linear graph for these phenomenon, and then work backwards to prove that aspirin is deadly, or that vitamins and minerals are deadly. Yet this same flaw in logic is often applied to radiation, and to radiation alone.

In fact, there are few, and perhaps none at all, linear dose-relationships in nature that hold true over all dosage scales.

Some scientists point out that when life first arose over 2 billion years ago, it evolved in an environment that had thousands of times more background radiation than we are exposed to today. This means that there must be much more room for life to live safely in a low radiation environment than once was previously imagined. None of this, of course, is meant to minimize the risks of higher levels of ionizing radiation.

Although exposure to ionizing radiation carries a risk, it is impossible to completely avoid exposure. Radiation has always been present in the environment and in our bodies. We can, however, avoid undue exposure.

Although people cannot sense ionizing radiation, there is a range of simple, sensitive instruments capable of detecting minute amounts of radiation from natural and man-made sources.

Dosimeters resemble pens, and can be clipped to one's clothing. They measure an absolute dose received over a period of time. They must be periodically recharged, and the result logged.

Time: For people who are exposed to radiation in addition to natural background radiation, limiting or minimizing the exposure time will reduce the dose from the radiation source.

Distance: In the same way that the heat from a fire is less intense the further away you are, so the intensity of the radiation decreases the further you are form the source of the radiation. The dose decreases dramatically as you increase your distance from the source.

Shielding: Barriers of lead, concrete, or water give good protection from penetrating radiation such as gamma rays and neutrons. This is why certain radioactive materials are stored or handled under water or by remote control in rooms constructed of thick concrete or lined with lead. There are special plastic shields which stop beta particles and air will stop alpha particles. Inserting the proper shield between you and the radiation source will greatly reduce or eliminate the extra radiation dose.

Shielding can be designed using halving thicknesses, the thickness of material that reduces the radiation by half. Halving thicknesses for gamma rays are discussed in the article gamma rays.

Containment: Radioactive materials are confined in the smallest possible space and kept out of the environment. Radioactive isotopes for medical use, for example, are dispensed in closed handling facilities, while nuclear reactors operate within closed systems with multiple barriers which keep the radioactive materials contained. Rooms have a reduced air pressure so that any leaks occur into the room and not out of it.

In a nuclear war, an effective fallout shelter reduces human exposure at least 1000 times. Most people can accept doses as high as 100 R, distributed over several months, although with increased risk of cancer later in life. Other civil defense measures can help reduce exposure of populations by reducing ingestion of isotopes and occupational exposure during war time. One of these available measures could be the use of potassium iodide (KI) tablets (http://www.radiation-pills.com) which effectively block the uptake of dangerous radioactive iodine into the human thyroid gland.